The Underboob vs Cleavage (NSFW)

I follow some “glamour” models on Instagram and cleavage is overrated when compared to the underboob. The underboob is looked down on because it’s showing a part of the boob that shouldn’t be shown. It doesn’t make a whole lot of sense though because with a push up bra you could probably show about 45% of the breasts, where as an underboob would only be 15%. The idea is that the nipple is always covered? It’s just a matter of time before girls not wearing shirts will be the same as guys not wearing shirts. I understand it’s to protect parts that are genetically different so people don’t feel ashamed, but nature is nature.